This project concerns the identification and kinetic description of the chemical events that intervene between the absorption of light and the initiation of electrical current flow in photoreceptors. The photoreceptors found along the ventral nerve of the horseshoe crab, Limulus polyphemus will be studied using intracellular microelectrode techniques. In this photoreceptor, responses to single photon absorptions can be recorded and are called discrete waves or quantal bumps. Discrete waves also occur spontaneously in the dark. Their frequency of occurrence is affected by temperature and by the concentration of calcium in the fluid bathing the photoreceptor. The molecular and physiological mechanisms that control the frequency of spontaneous discrete waves will be studied by measurements of the effects of temperature and calcium on discrete wave amplitude, frequency and waveform.